This invention relates generally to inflatable restraint systems and, more particularly, to an improved closure element for use with an installation of an inflatable cushion, commonly referred to as an air bag.
The value of safety restraint systems which self-actuate from an undeployed to a deployed state without the need for intervention by the operator, i.e., "passive restraint systems", and particularly those restraint systems incorporating inflatable bags or cushions has gained general appreciation.
It is well known to protect a vehicle occupant using a cushion or bag that is inflated with gas, e.g., an "air bag", when the vehicle encounters sudden deceleration, such as in a collision. During deployment, the rapidly evolving gas with which the bag is typically filled is an inert gas, e.g., nitrogen. In such systems, the cushion is normally housed in an uninflated and folded condition to minimize space requirements.
Upon actuation of the air bag system, gas is discharged from an inflator to rapidly inflate the bag. In general, such systems are designed to result in inflation of the air bag in a matter of a few milliseconds and with the bag thus serving to restrain the movement of the vehicle occupant as the collision proceeds.
For typical driver side inflators used in conventional air bag systems, gas pressure ranges between 250-650 kPa, as determined by tank testing wherein the inflator is actuated and exhausted into a one cubic foot volume tank with the pressure in the tank measured at fifty milliseconds after actuation. Those inflators that result in such gas pressures ranging from about 450-475 kPa up to about 650 kPa are hereinafter referred to as high pressure inflators.
Inflatable restraint systems have been devised for automotive vehicles in which one or more air bags are stored in one or more storage compartments within the vehicle. In general, an air bag provided for the protection of a vehicle driver, i.e., a driver side air bag, is stored within a module including an inflator, the air bag itself, a housing, a cover or some form of closure panel member, and attachment means mounted in the steering wheel of the vehicle. Whereas, an air bag for the protection of a front seat passenger, i.e., a passenger side air bag, is typically stored within a module mounted in the instrument panel/dash board of the vehicle. The covers of such installations commonly include a face portion which, in the standard state, provides closure to the assembly. Upon activation of the system and initial bag deployment, the cover typically forms one or more "doors" which hinge to form an opening through which the air bag will be deployed.
In order to improve the aesthetic appearance of the closure and to reduce the likelihood of tampering with the system, closure panel members are commonly designed so as to minimize the visual impression of the presence of the air bag and air bag deployment opening thereunder. Thus, such closure panel members are typically designed to match or otherwise be compatible with the interior design of the vehicle.
In addition, appearance or otherwise cosmetic problems such as closure read through and sink marks are also sought to be avoided. Read through (e.g., where shapes or features on the backside of the closure can be relatively easily discerned from a viewing of the front side of the closure) and sink marks (e.g., where the closure has an undesired, lower or uneven outer surface) can result from processing. For example, when the closure is prepared by molding fabrication of a thermoplastic material, an uneven shrinkage of the closure material upon processing can result in read through and/or sink marks. Such uneven shrinkage can, for example, occur as a result of the item being processed having relatively widely varying thicknesses- and thus varying processing time and condition requirements.
In practice, closure panel members are commonly designed so that the face portion of the closure can be torn or otherwise opened along predetermined lines as a result of the force exerted against the closure by way of the inflating air bag and, thus, form air bag release doors. To this end, the closure is typically pre-weakened, such as by perforating or reducing the thickness of the cover, for example, along such preformed tear lines, paths or seams, generally commonly referred to herein as "tear ways". It will be recognized that the tearing or opening of such closure panel members will typically follow the path of least resistance or highest stress and thus, typically result in the opening or tearing of the closure advancing along the thinnest or weakest path through the closure.
Furthermore, primarily to facilitate manufacture and operation, the face portion of prior art closures are commonly of uniform thickness except for the tear ways such as described above. Upon formation, the doors commonly remain joined to the closure via one or more hinge sections which permit the doors to move allowing the air bag to deploy through the resulting opening.
The component parts of such closures, fabricated from the thermoplastic rubber SANTOPRENE (a trademark of Monsanto Company), commonly have the following thicknesses:
______________________________________ PART THICKNESS (mm) ______________________________________ Door 2-3 Tear ways 0.4-1.1 Hinge 1.6-3 ______________________________________
As will be appreciated, the air bag deployment process is of necessity a very rapid process and, at least partially as a result thereof, commonly results in the door or doors opening with such force and energy that the closure can tear in an undesired and/or uncontrolled manner, such as tearing outside of the tear way such as into or through the hinge section joining the air bag deployment door with the assembly. In addition, once tearing of a part or a material has initiated, such tearing can be especially difficult to control or terminate as the tearing action, particularly in some plastic materials, once initiated has a tendency to propagate therethrough. Whereas undesired tear initiation in a part or a material is generally a more difficult proposition and occurs relatively infrequently.
Such undesired and/or uncontrolled tearing of the closure can result in the air bag closure breaking into or forming separate pieces. For example, should the tearing action proceed through the hinge section joining the door to the balance of the closure and/or assembly, the air bag release doors can become detached from the closure or the balance of the system installation or otherwise break into separate pieces. Such detached doors or fragments thereof could then undesirably come in contact with a vehicle occupant. Also, those closures used in conjunction with high pressure inflators may be more prone to detachment.
Various approaches have been employed in an attempt to overcome problems of excessive closure tearing, fragmentation and/or door detachment. Unfortunately, many of the approaches are so burdensome as to limit the flexibility and freedom in the designing of closures for specific installations and the operation thereof. In addition, many of the approaches invariably significantly increase the costs, e.g., manufacturing cost, production cost, etc., associated with such installations.
In one approach, strips or straps of material are used in the securing of the cover/door to the vehicle and/or assembly. In general, patents relating to such an approach specify the bolting, riveting or otherwise fastening of a first end of such strips or straps to the cover/door and similarly bolting, riveting or otherwise fastening an opposed second end of the strip or strap to the vehicle or to the reaction housing used in the air bag assembly. Examples of such U.S. Pat. Nos. include: 3,822,894; 4,893,833; and 5,150,919.
An approach which has found use in conjunction with high pressure inflator installations involves embedding a reinforcing band or web of a highly tear resistant material within a less tear resistant outer cover material. For example, closures used in conjunction with high pressure inflators are commonly constructed of reinforced urethane. Such reinforced urethane is typically in the nature of a reaction injection molded urethane, wherein a chemical reaction occurs in the mold during the fabrication and wherein a reinforcement insert such as of nylon is utilized.
U.S. Pat. Nos. 4,334,699 and 4,752,083 teach embedding a tear resistant band or a reinforcing element, respectively, in a rupturable cover. In general, the inclusion of such a secondary material within a closure almost invariably increases processing complexity, reduces design flexibility and increases cost.
Still another approach has been to vary or alter the shape, size, or dimensions of the tear or break lines of a closure such as fabricated of thermoplastic using a one-shot injection molding process. For example, U.S. Pat. No. 5,013,065 discloses the use of a rupturable cover having break lines of continuously increasing thickness. U.S. Pat. No. 3,622,176 discloses using V-shaped grooves along the fracture lines and hinge elements having rounded root grooves, so as to avoid fracture. U.S. Pat. No. 5,060,971 discloses using a cover whose thickness is specifically varied along the tear seam. Another example of such an approach is the use of a closure wherein the thickness of the tear ways is gradually increased as the longitudinal end of the tear way is approached.
Other or similar cover groove geometries are shown in other patents including U.S. Pat. Nos. 4,964,652; 5,002,307; 5,069,477; 5,087,071; and 5,143,401.
In addition, tear ways with graduated or steadily varying thickness have been used in an attempt to provide a more controlled tearing of the closure. While the utilization of designs having tear ways which vary in thickness typically provides greater control over the closure opening process including the site of the initial opening, such designs generally increase the likelihood of certain undesirable forms of closure tearing, such as tearing outside of the tear path. For example, when a closure is undergoing tearing along a tear path and the tearing action reaches a point in the tear way at which there is an abrupt increase in the thickness of the tear way, the abrupt change in thickness can result in tearing outside of the tear path. In the case of a tear way of gradually increasing thickness, as the thickness of the tear way increases, the difference in thickness of material at the tear way and the adjacent sections of the face portion diminishes, increasing the likelihood of non-tear way tearing of the closure.